I've been running ESXi 5.5 on my NUC for six months now, booted off USB3 key (Leaf Supra) and having its data on a internal spinning laptop drive (WD Red). Today I added a Crucial M550 mSATA drive and installed CentOS7 on that. Works great - Intel wifi/bluetooth card worked the first time, I'm even getting sound through HDMI under CentOS. Great stuff.

The issue I'm having is boot order. I have both UEFI and Legacy boot enabled, and have USB first and mSATA second in both categories in BIOS if you hit F2 to interrupt the bootup. The problem I'm seeing is that no matter what I do, the mSATA is booting regardless of being 'below' the USB disk in the order. Now where it really gets strange is if I powerup and hit F10 to manually select the boot, the order is very odd indeed - first pick is UEFI:SATA (CentOS) and the second is USB:LeafSupra (ESXi). In short, the F10 order is reversed.

I did check my bios rev and it's somehow 0021 (old) although the system was purchased only 6 months ago. Were there bios updates to fix boot order issues ? It's pretty close to impossible to decipher whether a bios update has something needed, and the big 'update only if it fixes a bug you have' disclaimer has me leery of messing with the bios needlessly.

The computer is acting like it's following the F10 ordering which is 'not' what the F2 ordering is. Disabling UEFI results in the system trying to PXE boot, skipping any disks entirely. Disabling Legacy and leaving UEFI enabled results in the system displaying >> Checking media presence >> media present >> start PXE off IPv4, again seeming to skip any disks.

So I think there's definitely a BIOS bug here. Manual boot of each disk works fine, so I know they are ok.

Update - did a F7 update to 0035 and see 'worse' behavior

now the system does not always see (nor light up) the USB disk every time, it seems slow to be recognized by the hardware. Pulling the power off the unit seems to help (ugh)

After doing so, the menu looks like:

UEFI : SATA : PORT 3 : CentOS

SATA : PORT 3 : Crucial

SATA : PORT 0 : WDC

LAN : IBA GE Slot 00C8

USB : Leef Supra

UEFI : USB : Leef Supra

what's interesting is the UEFI pick for the Leef Supra doesn't boot, you go back to the menu. Selecting the USB pick (second to last) does boot it.

As I indicated in the original post, the system booted hands-off to USB just fine before adding the mSATA drive,. It was only after the mSATA drive was added that the new drive seems to be out-prioritizing USB no matter what I do in the BIOS menus. Also the 0035 bios regressed and doesn't even see the USB disk all the time, which I think I've seen others complaining about in other threads.

I would be willing to snapshot every blasted screen if that's what is needed to get a solution here.